Cook the noodles according to package instructions, then drain and set aside.

In a small bowl, mix together soy sauce, sriracha, sesame oil, and rice vinegar.

In a wok or large skillet, heat a little oil over medium-high heat. Add the bell pepper, carrots, and broccoli, and stir-fry for about 5 minutes until tender.

Add the cooked noodles and protein (if using) to the vegetables in the wok. Pour the sauce over the top and toss everything together until well combined and heated through.

Garnish with chopped green onions and sesame seeds before serving.